Can you help me with my book character?

i have a book character that i really need help with, her name's Phasma, and the way i planned it, she was half human, half sorceress. she has a demon sealed inside of her, and so far, i had planned that her parents sealed it in her (not as cruel as it sounds). but i can't think of WHY they did it. so now i want to make her half demon on her dad's side, but i also wanted him to be part of this group that is the enemy of the Magi (that's what i call the magic people), and that was part of the original story. but i still kinda want her to be human, because it would seem too dramatic for a teenager to live thinking she was half human when she was really pert demon. how should i write this out?